Evolutionary stability of biological weed control

Reports of weeds rapidly evolving resistance to chemicals are common, but reports of weeds rapidly evolving resistance to biological control are rare. Possible reasons for the apparent evolutionary stability of weed biological control are: (1) lack of genetic variation, (2) genetic constraints on selection, (3) weak selection, (4) temporally varying selection, and (5) coevolutionary dynamics. Moreover it is not clear what traits of weeds might confer resistance to biological weed control. A review of the literature suggests the most likely explanation for the apparent evolutionary stability of weed biocontrol is (1) temporally variation in the selection imposed by biological control organisms, coupled with (2) robust time-averaging mechanisms of weed life histories including dormancy, perenniality, and iteroparity.